75000 - Document - Document and Entity Information link:presentationLink link:calculationLink link:definitionLink 75010 - Statement - STATEMENTS OF NET ASSETS AVAILABLE FOR BENEFITS link:presentationLink link:calculationLink link:definitionLink 75020 - Statement - STATEMENTS OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS link:presentationLink link:calculationLink link:definitionLink 75030 - Disclosure - Description of the Plan link:presentationLink link:calculationLink link:definitionLink 75040 - Disclosure - Significant Accounting Policies link:presentationLink link:calculationLink link:definitionLink 75050 - Disclosure - Fair Value Measurements link:presentationLink link:calculationLink link:definitionLink 75060 - Disclosure - Plan Termination link:presentationLink link:calculationLink link:definitionLink 75070 - Disclosure - Tax Status link:presentationLink link:calculationLink link:definitionLink 75080 - Disclosure - Party-In-Interest Transactions link:presentationLink link:calculationLink link:definitionLink 75090 - Disclosure - Subsequent Events link:presentationLink link:calculationLink link:definitionLink 75100 - Disclosure - Significant Accounting Policies (Policies) link:presentationLink link:calculationLink link:definitionLink 75110 - Disclosure - Fair Value Measurements (Tables) link:presentationLink link:calculationLink link:definitionLink 75120 - Disclosure - SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Tables) link:presentationLink link:calculationLink link:definitionLink 75130 - Disclosure - Description of the Plan (Additional Information) (Details) link:presentationLink link:calculationLink link:definitionLink 75140 - Disclosure - Fair Value Measurements - Schedule of Fair Value of Plan's Assets (Details) link:presentationLink link:calculationLink link:definitionLink 75150 - Disclosure - Tax Status - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75160 - Disclosure - Party-In-Interest Transactions - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75170 - Disclosure - SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Details) link:presentationLink link:calculationLink link:definitionLink 75170 - Disclosure - SCHEDULE H, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Details) [Default] link:presentationLink link:calculationLink link:definitionLink Additions to net assets attributed to EBP, Change in Net Asset Available for Benefit, Increase [Abstract] Benefits paid to participants EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Fair Value, Inputs, Level 2 [Member] Level 2 EBP, Related Party and Party-in-Interest Transactions [Line Items] Document Fiscal Period Focus EBP, Forfeited Nonvested Account, Decrease from Employer Contribution Future employer contributions reduction EBP, Tax Status [Abstract] EBP, Forfeited Nonvested Account Employer contribution reduction EBP, Description of Plan [Text Block] Description of the Plan EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Interest income on notes receivable from participants Investment, Identifier [Axis] EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Administrative expenses EBP, Description of Plan [Line Items] EBP, Debt, Interest Rate Interest rate EBP, Investment, Issuer Name [Extensible Enumeration] EBP, Investment, Fair Value and NAV [Abstract] EBP, Accounting Policy [Line Items] Investment, Name [Axis] EBP, Schedule of Asset Held for Investment [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Maximum borrowing amount Common Collective Trust [Member] Common/Collective Trust Funds Amendment Flag EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] Deductions to net assets attributed to EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount Minimum borrowing amount EBP, Payment to Participant [Policy Text Block] Distributions Residence [Member] Residence. Residence EBP, Schedule of Asset Held for Investment [Line Items] Fair Value Hierarchy and NAV [Axis] Financial Institutions, Inc. Common Stock EBP, Employer, Common Stock Fund [Member] EBP, Investment, Current Value, Type [Extensible Enumeration] Investment Type [Axis] Document Type Entity Central Index Key EBP, Forfeited Nonvested Account, Decrease from Administrative Expense Plan administrative expense EBP 002 [Member] EBP 002. EBP 002 EBP, Investment [Policy Text Block] Investment Valuation and Income Recognition EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Other Investment Income (Loss) Other income EBP, Contribution, Participating Employer [Axis] EBP, Subsequent Event [Table] EBP, Accounting Policy [Table] EBP, Subsequent Event [Abstract] EBP, Basis of Accounting [Policy Text Block] Basis of Accounting Current Fiscal Year End Date EBP, Description of Plan [Table] Investment, Name [Domain] Common Stock EBP, Employer, Common Stock [Member] EBP, Investment, Number of Shares Minimum [Member] Minimum EBP, Related Party and Party-in-Interest Transactions [Text Block] Party-In-Interest Transactions EBP, Change in Net Asset Available for Benefit, Increase Total additions Fair Value, Inputs, Level 3 [Member] Level 3 EBP, Change in Net Asset Available for Benefit, Increase from Contribution Total contributions EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] Employee Benefit Plan ERISA Account Balance Employee benefit plan ERISA account balance. ERISA account balance EBP, Investment Sold, Excluding Plan Interest in Master Trust, Selling Price Sales made by the plan EBP, Accrued Expense Plan expense Fair Value Measured at Net Asset Value Per Share [Member] Fair Value Measured at Net Asset Value Per Share EBP, Statement of Net Asset Available for Benefit [Abstract] EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Participant EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] Contributions: EBP, Tax Determination Letter, Date Tax opinion letter, date EBP, Participant Contribution, Automatic, Deferral Rate Deferral rate EBP, Contribution, Participating Employer [Domain] EBP, Related Party and Party-in-Interest Transactions [Abstract] EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value Total investments Investments at fair value Use of Estimates EBP, Use of Estimate [Policy Text Block] Mutual Fund [Member] Mutual Funds Entity [Domain] EBP, Investment Acquired, Excluding Plan Interest in Master Trust, Cost Purchases made by the plan EBP, Summary of Accounting Policy [Text Block] Significant Accounting Policies Employee Benefit Plan, Note Receivable from Participant, Account, Repayment Period Maximum term of participant loan repayment. Repayment term of participant note EBP, Investment, Fair Value and NAV [Text Block] Fair Value Measurements EBP, Description of Plan and Accounting Change [Abstract] EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution EBP, Statement of Change in Net Asset Available for Benefit [Line Items] Statistical Measurement [Domain] EBP, Note Receivable from Participant Notes receivable from participants Document Period End Date Fair Value Hierarchy and NAV [Domain] EBP, Risk and Uncertainty [Policy Text Block] Risks and Uncertainties EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage Percentage of maximum borrowing amount Tax opinion letter, obtained EBP, Tax Determination Letter, Obtained [true false] EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Plan Termination [Text Block] Plan Termination EBP, Subsequent Event [Text Block] Subsequent Events Cover [Abstract] Investments [Domain] EBP, Net Asset Available for Benefit Net assets available for benefits at beginning of year Net assets available for benefits at end of year Net assets available for benefits Schedule of Fair Value of Plan's Assets EBP, Investment, Fair Value and NAV [Table Text Block] Cash And Cash Equivalent [Member] Cash and cash equivalent. Cash and Cash Equivalents EBP, Schedule of Asset Held for Investment [Abstract] EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value EBP, Note Receivable from Participant [Policy Text Block] Notes Receivable from Participants Contribution description EBP, Employer Contribution, Noncash, Description EBP, Schedule of Asset Held for Investment [Table Text Block] Schedule of Assets (Held at End of Year) EBP, Statement of Net Asset Available for Benefit [Table] Document Fiscal Year Focus EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment, Total Net appreciation in fair value of investments EBP, Statement of Change in Net Asset Available for Benefit [Table] Employee Benefit Plan, Retired Or Separated Participant, Vested Account Balances Distribution Amount of vested account balances distributed to particpant upon termination of employment, disability, or retirement or to the participant's estate in the case of death. Employee benefit plan, participant termination, vested account balances distributed Contributions EBP, Contribution [Policy Text Block] EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Percentage on employee's eligible compensation EBP, Plan Number EBP, Investment, Fair Value and NAV [Line Items] Legal Entity [Axis] EBP, Plan Name EBP, Asset [Abstract] ASSETS EBP, Investment, Fair Value and NAV [Table] EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Rollovers EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] EBP, Asset Held for Investment EBP, Asset Held for Investment, Total EBP, Subsequent Event [Line Items] EBP, Tax Status [Text Block] Tax Status Maximum [Member] Maximum Statistical Measurement [Axis] EBP, Statement of Change in Net Asset Available for Benefit [Abstract] Employee Benefit Plan Maximum Period for Distribution of Investments Employee benefit plan maximum period for distribution of investments. Maximum term of benefit plan Entity Registrant Name EBP, Asset Held for Investment, Participant Loan, Interest Rate Non participating employee Non Participating Employee [Member] Non participating employee. Percentage of common stock plan's investment EBP, Investment, Employer, Common Stock, to Total Asset, Percentage EBP, Change in Net Asset Available for Benefit, Decrease Total deductions Fair Value, Inputs, Level 1 [Member] Level 1 EBP, Tax Status [Line Items] Entity Tax Identification Number EBP, Asset Held for Investment, Participant Loan, Maturity Date EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Net increase Net increase EBP, Tax Status [Table] EBP, Description, Available [true false] EBP, Asset Held for Investment, Party-in-Interest [true false] Vanguard total bond market index fund admiral shares. Vanguard Target Retirement 2070 Fund. Vanguard Target Retirement Income [Member] Vanguard Target Retirement 2070 Fund [Member] Vanguard target retirement income. Vanguard Target Retirement 2025 Fund [Member] Vanguard target retirement 2055 fund. Fidelity large cap growth index fund. Vanguard Mid-Cap Index Fund Admiral Shares [Member] Vanguard 500 index fund admiral shares. Vanguard target retirement 2020 fund. Vanguard Target Retirement 2055 Fund [Member] Baird Core Plus Bond Fund; Institutional Class [Member] Vanguard mid-cap index fund admiral shares. Vanguard target retirement 2065 fund. Vanguard Target Retirement 2020 Fund [Member] Vanguard small-cap index fund admiral shares. Putnam Large Cap Value Fund; Class R6 [Member] American Funds EuroPacific Growth Fund; Class R-6 [Member] Vanguard Target Retirement 2065 Fund [Member] Vanguard Federal Money Market Fund [Member] Vanguard Small-Cap Index Fund Admiral Shares [Member] American funds europacific growth fund; class r-6. Putnam large cap value fund; class r6. Vanguard Target Retirement 2060 Fund [Member] Vanguard target retirement 2045 fund. Vanguard Target Retirement 2045 Fund [Member] Vanguard Target Retirement 2030 Fund [Member] Vanguard 500 Index Fund Admiral Shares [Member] Vanguard Retirement Savings Trust III [Member] Vanguard federal money market fund. Vanguard Emerging Markets Stock Index Fund Admiral Shr [Member] Vanguard Target Retirement 2040 Fund [Member] Vanguard target retirement 2040 fund. Vanguard target retirement 2030 fund. Financial institutions, INC. Company stock. Vanguard target retirement 2050 fund. Vanguard Total Bond Market Index Fund Admiral Shares [Member] Vanguard Target Retirement 2035 Fund [Member] Vanguard retirement savings trust III . Vanguard emerging markets stock index fund admiral shr. Vanguard FTSE all-world ex-us index fund admiral shares. Vanguard target retirement 2060 fund. Fidelity Large Cap Growth Index Fund [Member] Vanguard Target Retirement 2050 Fund [Member] Baird core plus bond fund; institutional class. Vanguard FTSE All-World ex-US Index Fund Admiral Shares [Member] Financial Institutions, Inc. Company Stock [Member] Vanguard target retirement 2025 fund. Vanguard target retirement 2035 fund.